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Lecture 2
What is a star?
A huge ball of ionized gas (plasma) producing energy (light) by nuclear fusion.
The visible surface of the sun (photosphere) is ~5700K. An object at this temperature emits most of its
energy as visible light
Light is electromagnetic radiation emitted at various wavelengths
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Because of its electric and magnetic properties, light is also called electromagnetic radiation
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Visible light falls in the 400 to 700 nm range
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Stars, galaxies and other objects emit light in all wavelengths
An opaque object emits electromagnetic radiation
according to its temperature
Experiments show that solids, liquids, and dense gases (like our sun) emit continuous spectra due to
temperature – thermal or blackbody emission
Experiments show that dilute gases produce either emission or absorption lines instead of a continuous
spectrum
Experiments with gases and prisms showed that substances can absorb parts of any light that shines
on them. These substances can re-emit that absorbed light at a later time. An emission line spectrum
is equivalent to thermal emission by the cloud of gas.
Spectral lines are produced when an electron jumps from one energy level to another within an atom
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The nucleus of an atom is surrounded by electrons that occupy only certain orbits or energy
levels
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When an electron jumps from one energy level to another, it emits or absorbs a photon of
appropriate energy (and hence of a specific wavelength).
The spectral lines of a particular element correspond to the various electron transitions between energy
levels in atoms of that element.
Each chemical element produces its own unique set of spectral lines
Hydrogen lines will be the most important for stars
Photosphere is a thin layer (~300-500 km thick) of the sun which is opaque – so is “visible surface” of
sun. Temp of photosphere ranges from ~4000-8000K, but mostly is 5700K (~10,000F). When we talk
about the “temperature of a star”, we mean its photospheric temperature
Limb darkening – at limb, light is coming
from upper part of photosphere, not base
On large scale, sun produces continuous spectrum. Photosphere is cooler than interior – so see faint
absorption lines (Fraunhofer lines) – use to determine chemical composition of photosphere (and
presumably whole sun). Note: for all stars, the temperature and chemistry determined from spectra
is for PHOTOSPHERE. Interior is theory.
We can figure out the chemical composition of the sun by matching each of the sun’s absorption lines to
a spectra measured in a laboratory for a particular element or molecule
A star’s color depends on its surface temperature
The spectra of stars reveal their chemical
compositions as well as surface temperatures
The “Harvard Classification” of stars which we use today was generated by teams of women
astronomers in the late 1800s (who were paid less than Harvard’s secretaries). They sorted stellar
spectra into 7 classes based on color (and thus temperature) and spectral features
OBAFGKM
Hottest ----------------------Coolest
L = σT4A, where A = 4πr2
L = brightness (luminosity) of star
σ = a constant
T = temperature
A = area which depends on two contants (4 and π) and the radius (size)
of the star)
So, a bright star can be HOT, or it can be COOL BUT VERY VERY BIG
Two guys named Hertzsprung and Russell compare the spectral class (temperature) and brightnesses
(luminosity) of all the stars for which they had data – the diagram they created is called an H-R diagram
What they found is that most stars fall in a narrow band called the “main sequence”, but that some stars
were big but cool or small but hot. Why?
To understand the H-R diagram, you need to look at the life cycle of stars
Stars are born, they live for a while, then they die – in the process, they create the chemical elements of
the periodic table.
The life of a star is a constant battle against gravity – and gravity always wins
Stars form from immense regions of gas and dust called a nebula (plural nebulae or nebulas
It’s all about MASS
Anything with mass is affected by gravity
Gravity tries to pull objects of mass towards each other
In a gas (nebula), THERMAL PRESSURE is what keeps atoms and molecules from all moving towards
the center of mass
What do we mean by thermal pressure?
In a gas, the molecules are always in motion. The hotter the gas, the faster the motion, with many
molecules trying to move away from the center (the gas tries to expand).
When a gas is compressed, it heats up, and so its thermal pressure increases.
If a nebula is not actively contracting or expanding, then gravity is in balance with thermal pressure
Need a trigger (perturbation) to cause nebula to collapse (feel more gravity than thermal pressure)
Triggers include:
Supernova
Collisions between nebulae
Solar wind pressure from nearby O and B stars
Other? Anything that disturbs balance
In order to be dense enough to collapse from a slight perturbation, need to be molecular cloud
Stages:
Free-fall contraction
Kelvin-Helmholtz contraction – slows contraction
Protostar or YSO before it reaches main sequence
Center of protostar becomes hot enough to glow in visible at some part – cool, but very large, so very
luminous – HOWEVER, surrounded by exterior of protostar, so we don’t actually see star at this point
Different types of YSOs:
1) IR stars
2) Proplyds
3) Herbig-Haro objects
4) T-Tauri stars
Range from protostars
to newly-formed stars
IR stars:
infrared protostars
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show excess emission in IR compared to visible wavelengths
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explained by dust cocoons surrounding protostar
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these cocoons obscure the visible light from the central star-like object, but are warmed by
that visible light and so radiate in the IR
Proplyds: protoplanetary disks
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dusty cocoons seen around YSOs
first seen clearly in the Orion Nebula
Herbig-Haro
Objects-•
YSOs with
disks & bipolar
outflows
Solar Nebula - a spinning disk of gas and dust surrounding the newly forming sun.
T-Tauri star:
catchall term for many types of YSOs
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YSO has not yet achieved H to He fusion and is not yet on the “Main Sequence” trend of
luminosity & temperature
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can show evidence for rapid rotation, strong magnetic field, strong stellar winds, short-lived
brightness outbursts (FU-orionis outbursts), excess IR emission
Eventually interior of star becomes hot enough for nuclear fusion – star becomes main sequence star
Nebulae are huge – as they collapse, they break into numerous fragments – each of which will
become a star system – star must be between ~0.1 solar masses to ~60 solar masses.
So tend to form CLUSTER of stars
High mass stars take less time to go from start of collapse to main sequence than do low mass stars
Eventually interior of star becomes hot enough for nuclear fusion – star becomes main sequence star
– either proton-proton chain or CNO cycle
CNO cycle – more efficient method, but requires higher internal temperature, so only for stars with
mass higher than ~1.1 solar masses
12C + p -> 13N
13N -> 13C
13C + p -> 14 N
14N + p -> 15O
15N + p -> 12C + 4He
15O -> 15N
Net reaction: 4 hydrogen converted to 1 helium
A more massive star must produce more energy to support its own weight – reason there is a
correlation
of mass and luminosity on main sequence
Internal structure of main sequence star varies with mass
Reminder – Life Cycle thus far:
Nebula  collapse
Protostar YSO, Herbig Haro, T-tauri
Main Sequence
Main sequence stars have mass > 0.1 solar masses
and < 60 solar masses
Main sequence – thermal pressure from hydrogen
fusion in core (either proton proton chain
or CNO cycle) – balances gravity
Mass-luminosity relationship – in order to balance gravity,
must have
L = M3.5
So a 4 solar mass star will be 128 times brighter than sun
Lifetime on main sequence = fuel/rate of consumption
= M/L = M/M3.5 =
1/M2.5
So a 4 solar mass star will have a main sequence lifetime 1/32 as long as our sun
What do we mean by “lifetime”? The length of time over which there is Hydrogen available in the core
Eventually, all of the material in the core in converted to Helium and all of the Hydrogen is outside the
core and too cool to fuse.
The Sun has been a main-sequence star for about 4.56 billion years and should remain one for about
another 7 billion years
During a star’s main-sequence lifetime, the star expands somewhat and undergoes a modest increase in
luminosity
This is because we’ve converted part of the core into Helium, the core has contracted, causing the gas to
heat up, heating and expanding the gas outside the core
So main sequence is not a line on an H-R diagram, but a band
So, what happens when the core runs out of hydrogen?
No fusion = thermal pressure doesn’t balance gravity = star begins to collapse (contract) = get additional
heating by Kelvin-Helmholtz contraction
Additional heat from contraction raises temperature of a shell of hydrogen outside core to high enough
for hydrogen fusion – “shell burning”
Fusion of 4 hydrogen to form helium in a shell provides enough heat to support outer layers of star, but
doesn’t do anything for core
Core keeps shrinking – producing heat from contraction (way above amount need to fuse hydrogen, but
there is no hydrogen in core). This heat is added to that of “shell burning”, so outer layers of star have
more thermal pressure than needed to balance gravity
Outer layers of star expand while core continues to contract  RED GIANT
When core hydrogen fusion ceases, a main-sequence star becomes a red giant
Concept – ideal gas – pressure depends on temperature (PV=nRT), so hot gas has higher pressure (and
expands)
Electron degeneracy – we have an ionized gas (electrons not attached to nuclei) - gravity pushes
electrons and nuclei close together – electrons cannot get any closer together without having same
energy (violating pauli exclusion principle).
An electron degenerate gas will not expand as temperature increases
STELLAR LIFECYCLE:
What happens next depends on mass
For stars < 0.4 solar masses, core stops contracting due to degeneracy pressure before becoming hot
enough to fuse He.
For stars > 3 solar masses, core becomes hot enough to fuse He before matter becomes degenerate.
In between, stars become degenerate before getting hot enough to ignite He  Helium Flash
Helium Flash – core gets hot enough to fuse He, but doesn’t expand (so not offsetting gravity), so fuses
faster and faster, until temperature becomes so high that electrons can move away from nuclei and are
no longer degenerate. Star’s core rapidly expands (helium flash) and then settles down to steady rate of
helium fusion – can lose outer layer of tenuous stellar atmosphere in a ring called a planetary nebula
Helium fusion – 3 helium nuclei fuse to form one carbon nuclei (triple alpha process)
4
He + 4He  8Be + gamma rays
8
Be + 4He  12C + gamma rays
Expansion of core absorbs some of energy that went to expanding the outer layers, so outer layers
shrink slightly
Can use turnoff point for a star cluster to determine age of cluster
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A low-mass star becomes
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a red giant when shell hydrogen fusion begins
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a horizontal-branch star when core helium fusion begins
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an asymptotic giant branch (AGB) star when the helium in the core is exhausted and
shell helium fusion begins
Dredge-ups bring the products of nuclear fusion
to a giant star’s surface
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As a low-mass star ages, convection occurs over a larger portion of its volume
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This takes heavy elements formed in the star’s interior and distributes them throughout the star
Low-mass stars die by gently ejecting their outer
layers, creating planetary nebulae
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Helium shell flashes in an old, low-mass star produce thermal pulses during which more than
half the star’s mass may be ejected into space
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This exposes the hot carbon-oxygen core of the star
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Ultraviolet radiation from the exposed core ionizes and excites the ejected gases, producing a
planetary nebula
The burned-out core of a low-mass star cools
and contracts until it becomes a white dwarf
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No further nuclear reactions take place within the exposed core
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Instead, it becomes a degenerate, dense sphere about the size of the Earth and is called a white
dwarf
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It glows from thermal radiation; as the sphere cools, it becomes dimmer
High-mass stars create heavy elements in their cores
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Unlike a low-mass star, a high mass star undergoes an extended sequence of thermonuclear
reactions in its core and shells
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These include carbon fusion, neon fusion, oxygen fusion, and silicon fusion
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In the last stages of its life, a high-mass star has an iron-rich core surrounded by concentric
shells hosting the various thermonuclear reactions
The sequence of thermonuclear reactions stops here, because the formation of elements heavier than
iron requires an input of energy rather than causing energy to be released
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A high-mass star dies in a violent cataclysm in which its core collapses and most of its matter is
ejected into space at high speeds
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The luminosity of the star increases suddenly by a factor of around 108 during this explosion,
producing a supernova
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The matter ejected from the supernova, moving at supersonic speeds through interstellar gases
and dust, glows as a nebula called a supernova remnant
Lots of stuff without text
So why are we talking about star formation and nucleosythesis?
Because we think the universe had a beginning.
Our current understanding of the origin of the universe indicates that the only elements that formed
were Hydrogen and Helium, with perhaps small amounts of Li, Be, and B. You can’t make planets or life
from just this.
The very first stars would have had no heavy elements, and they would have been massive – so they are
exactly the type of stars that are born quickly, live fast, and die spectacularly, spewing heavier elements
out into the universe via supernova explosions. Lower mass stars would have formed in a region already
contaminated by the material provided by the deaths of these higher mass first generation stars.
The lowest mass (M stars) have main sequence lifetimes longer than the age of the universe—every one
that has ever formed is still with us today as a main sequence star
Type I = population I star (chemically like our sun)
Type II = population II star (fewer “metals” than our sun)
We see these older stars—they have less heavy elements than our sun
If we look at star forming regions today (such as the Orion nebula), we notice that the new stars have
more heavy elements than our sun.
Without the life and death of stars, there would be no rocky material from which to make terrestrial
planets, there would be no oxygen to make water, and there would be no carbon, nitrogen, oxygen, or
other materials with which to make life – we wouldn’t be here to take this class.